String Walker

String Walker

It has often been suggested that the best locomotion mechanism for virtual
worlds would be walking. It is well known that the sense of distance or
orientation while walking is much better than that while riding in a vehicle.
However, the proprioceptive feedback of walking is not provided in most
applications of virtual environments. We have been developing various
prototypes of interface devices for walking since 1989, including torus-shaped
omni-directional treadmill, motion foot pad, and robot tiles. These locomotion
interfaces create infinite surface by the use of motion floors. Realization of
the motion floors needs bulky or complex drive mechanism, which restricts
practical use of locomotion interfaces. We therefore started projects to
develop locomotion interface using actuated shoes instead of motion floors.
The Powered Shoes, which we demonstrated in SIGGRAPH2006, was a first example.
It is a light weight wearable device but has several limitations in its
function and durability. Considering these limitations, we developed a new
locomotion interface named “String Walker”. It employs actuated shoes using
motor-driven strings. The goal of this project is to develop a string-based
locomotion interface that enables the user to omni-directional walking in
various gaits, while the position is maintained. In order to achieve this
goal, tension of the strings must be effectively generated. Research on
locomotion interfaces is still in a preliminary state, but some applications
of virtual environment, such as training or visual simulation needs a good
sensation of locomotion. The next decade will see effective devices of
locomotion interface in these applications.

2 Technical Innovation of the Project

2.1 Basic design

The major innovation of this work is a new actuation mechanism that cancels
the displacement of the walker. Existing locomotion interfaces employ motion
floors for creation of infinite surface. The easiest way to realize an
infinite floor is the use of a treadmill [1][2][3][4]. However, a treadmill
has difficulty in realizing omni-directional walking [8][13]. Motion foot-pad
for each foot is an alternative [14]. It has ability to simulate
omni-directional walking as well as walking on uneven surface. The major
limitation of this method is that severe accuracy is required for the foot-pad
to trace the walker. Actually, the walker has to be careful about miss tracing
of the foot-pad. The CirculaFloor was developed to overcome drawbacks of
treadmills and foot-pads [15]. However, the system is too complicated to
achieve sufficient walking speed. The Powered Shoes employs roller skates
instead of motion floor [16]. The roller skates are actuated by motors and
flexible shafts. The major limitation in the system is that the direction of
the traction force generated by the rollers is identical to the shoe, which
only allows the walker in a straight gait. The String Walker is a new
locomotion interface that employs motor-driven strings. Four strings are
connected to each shoe and they are actuated by motor-pulley mechanisms. Each
motor is equipped with rotary encoder and the motor-pulley mechanisms can
measure position and orientation of the shoe. The strings pull the shoe in
opposite direction of walking, so that the step is canceled. The position of
the walker is fixed in the real world by this computer-controlled tension of
the strings. The motor-pulley mechanisms are mounted on a motor-driven
turntable. It rotates according with the direction of the walker, which
enables omni-directional walking. Figure 1 illustrates basic structure of the
system and Figure 2 shows its overall view. The four strings can pull the shoe
in omni-direction, which enables the walker to various gaits such as
side-walking or backward walking. This is the major advantage of the system
compared to the Powered Shoes that we demonstrated in the SIGGRAPH 2006.

Figure 1. Basic structure of the string-based system
Figure 2. Overall view of the String Walker

2.2 Hardware configuration

Four strings are connected to each shoe (Figure 3). These
strings apply force to the shoe in arbitrary direction. A motor-pulley
mechanism generates tension of the string (Figure 4). It also measures
position and orientation of the shoe. The maximum tension of each string is
25Kgf. A touch sensor is equipped at the shoe (Figure 5). It detects stance
phase and swing phase of walking. The signal is wirelessly transmitted to the
host computer. The tension is not applied to the shoe while it is in the swing
phase. The motor-pulley mechanisms are mounted on a turntable driven by a
motor (Figure 6). When the walker changes direction of walking, the turntable
is activated to follow the direction of walker. This function enables
omni-directional walking. Diameter of the turntable is 1800mm.

Figure3.Strings attached to the shoes
Figure4. Motor-pulley mechanism

Figure5. Touch sensor
Figure 6. Motor-driven turntable

2.3 Method of walking

The user’s position in virtual space is updated corresponding
to the results of motion tracking of the feet. Figure 7 illustrates basic idea
of the detection of walking. A circular dead zone is placed in the center of
the walking area. In this study, the dead zone is 200 mm diameter circle. The
positions of the walker’s feet are measured with motor-pulley mechanisms as
described in the previous section. Point G in Figure 7 represents the
projection of the central position of the walker. The strings don’t apply
force while the point G is inside the dead zone. If the point G leaves the
dead zone, the strings pull the foot so that the point G returns to this area.
The direction and distance between the point G and the circle determines the
pulling back direction and velocity of the shoe respectively. The control
algorithm must keep the position of the walker at the central position of the
String Walker system. In order to keep the position maintained the strings
have to cancel the motion of the feet. Figure 8 illustrates methods of
cancellation. The principal of the cancellation is: 1) Suppose the left foot
is at the forward position and right foot is at the backward position while
walking. 2) When the walker steps forward the right foot, the weight of the
walker is laid on the left foot. 3) The strings pull the left foot back in
accordance with the displacement of the right foot, so that the position of
the walker is maintained. The eight strings can generate force in arbitrary
direction so that they can cancel various gait including backward walking and
side stepping. Figure 9 illustrates cancellation method for side stepping.

Figure 7. Detection of walking

Figure 8.Cancellation of walking
Figure 9. Cancellation of side stepping

3 Experience for the SIGGRAPH 2007
attendees

The experience of the String Walker is simple. The attendees only put the
shoes on and walk. The participant of the system can enjoy omni-directional
walking while his/her position is localized in the real world. We can
demonstrate the performance of the system in 3 minute. One person can
participate at one time, but waiting people can enjoy seeing motion of the
shoes. Considering individual differences, we are going to prepare several
sizes of the shoes. Since most of SIGGRAPH attendees are novice user of the
system, a safety bar mounted on the turntable will be provided to support the
walker. Setup of the system is easy. We bring all the apparatus from Japan.
Only turntable is set on the floor of the booth. It will need half day for
setup. Required space is 13ft X 20ft.

4 How does this work expand on prior
work?

We have been developing prototypes of locomotion interfaces for virtual
environments since 1989. The first project was named Virtual Perambulator[12].
A user of the system wore parachute-like harness and omni-directional roller
skates. The trunk of the walker was fixed to the framework of the system by
the harness. Omni-directional sliding device is used for changing direction by
feet. We developed specialized roller skate equipped with four casters which
enabled two-dimensional motion. The walker could freely move his/her feet in
any direction. This is the first locomotion interface in the world. Later, the
harness was removed and a hoop was set around the walker's waist in which
he/she can physically walk and turn about. We demonstrated it at the
Interactive Communities venue in the SIGGRAPH'95. During five days conference,
235 people experienced the device. We observed behavior of the walkers and 94%
of them succeeded in rhythmical walking. However, walkers had to slide their
feet by themselves. In other words, the device was passive. Walkers had to get
accustomed to the sliding action. We therefore aimed to develop an active
device which moves corresponding to motion of the walker. In order to achieve
an omni-directional active floor, we developed a specialized treadmill named
"Torus Treadmill" in 1997[13]. The device employed 12 treadmills connected
side-by-side. They rotate in perpendicular direction. Thus, each treadmill
cancels back-and-forth motion of the walker, and rotation of 12 treadmills
cancels his/her left-and-right motion. The device enables natural walking on a
flat surface. However the hardware is very large and complex. We could not
bring it out from the laboratory. In 1999, we developed a new locomotion
interface named "GaitMaster[14]." It was equipped with two motion platforms to
support the walker's feet. The foot-pads trace position of the feet to provide
an infinite floor. The device could simulate uneven surface such as
staircases. We managed to demonstrate it at the Emerging Technologies venue of
the SIGGRAPH 2000. The demonstration was successful but the device has
limitation in tracing accuracy of the feet. We therefore put safety strap to
prevent the walker to fall off from the foot-pad. The CirculaFloor, developed
in 2002, takes advantage both from treadmill and foot-pad [15]. It creates
omni-directional infinite surface by the use of a group of movable tiles.
Combination of the tiles provides sufficient area for walking, thus precision
tracing of the foot position is not required. The CirculaFloor was
demonstrated at the Emerging Technologies venue of the SIGGRAPH 2003. The
system requires high-level implementation of actuation mechanism of the tiles
and wireless controller. It has difficulty in improving its performance. The
Powered Shoes, which we demonstrated in SIGGRAPH2006, is a light weight
wearable device. It has several limitations in its function and durability.
Traction of the roller skate is limited in one direction. Thus gait of the
walker is limited. Side stepping is not allowed, for example. Another problem
in the Powered Shoes is durability of the flexible shafts. They were
frequently broken during the SIGGRAPH demo. The flexible shafts are
specialized parts so that we had problems in logistics. Based on these
experiences in locomotion interface, we propose the String Walker to solve all
the difficulties in implementation as mentioned above.

5 Larger implications of the project beyond
this demonstration phase

Walking on foot is the most intuitive way to move about the real world.
Although advanced visual simulation often requires good sense of locomotion,
existing systems do not provide sense of walking. The String Walker is a
practical solution to allow the user to natural walking. It has wide
application areas including training simulators or entertainment. One of the
serious applications will be training simulator for dismounted infantry. The
String Walker allows the user to various gaits including side-stepping or
backward walking. These capabilities contribute to training in urban space or
buildings. Another serious application may be an "evacuation simulator."
Analysis of evacuation of people in disasters is important in social safety.
However, it is impossible to carry out experiments with human subjects during
an actual disaster. Virtual environment is inevitable for such experiments.
Since evacuation is done by walking or running, the String Walker will be an
indispensable interface device for the experiments. Combination of the String
Walker and an immersive image display may provide ultimate sense of presence.
The integrated system can greatly contribute to teleoperation or virtual
travel.